Parts for your 2017 Subaru Legacy-Oxygen sensor

2017 Subaru Legacy oxygen sensor: what it does and when to service it

The 2017 Subaru Legacy absolutely uses oxygen-sensing hardware. Per Subaru’s 2017 Legacy/Outback Factory Service Manual and parts catalogue, the petrol FB25 2.5‑litre has a front air–fuel ratio (A/F, wideband) sensor before the catalytic converter and a rear oxygen sensor after it, the 3.6R (EZ36) runs one A/F sensor and one downstream oxygen sensor per bank. That layout also aligns with OBD‑II requirements (SAE/ISO standards) that mandate upstream and downstream sensing for fuel control and catalyst monitoring.

On this model, the front A/F sensor is the maestro of fuel trim. It constantly reads oxygen in the exhaust so the ECU can hold a stoichiometric mixture, keeping performance smooth and fuel economy tidy. The rear oxygen sensor lives after the cat and keeps an eye on catalyst efficiency, it helps the ECU detect if the converter is doing its job.

These sensors aren’t typically a scheduled replacement item, but they do age. Many workshops treat 150,000–180,000 km as a sensible window to evaluate them, or they’re replaced when faults appear.

Common signs it’s time to check the oxygen sensor set on a 2017 Legacy:

• Check Engine light with codes such as P0131/P0134/P0137/P0141 or P0420
• Heavier fuel use, rough idle, or flat spots
• Emissions test failures or sulphur/rotten‑egg smells

Service tips owners appreciate:

• Know which is which: the front sensor is a wideband A/F unit, the rear is a conventional O2 sensor. They’re not interchangeable.
• Inspect wiring and connectors during routine services, heat and road grime are hard on them.
• Use quality, correct‑spec sensors. Many genuine and reputable aftermarket sensors come with anti‑seize on the threads—avoid getting any on the sensing tip.
• Fix exhaust leaks and vacuum leaks promptly, both can skew readings and trigger false faults.
• After replacement, clear codes and complete a proper drive cycle so the ECU can relearn trims and run catalyst tests.

For the 3.6R, there are four sensors (two banks), for the 2.5i there are two. A technician referencing the Subaru service manual procedures will confirm placement and test values before calling a sensor faulty, as issues like a tired catalytic converter or wiring damage can mimic a bad sensor.

Popular questions

How many oxygen sensors does a 2017 Subaru Legacy have?
The 2.5‑litre FB25 typically has two: one upstream A/F (wideband) sensor and one downstream oxygen sensor. The 3.6R EZ36 has four, with an A/F and a downstream oxygen sensor on each bank. Subaru’s 2017 service documentation and parts listings reflect this layout.

When should the oxygen sensor be replaced?
There’s no fixed time‑based interval in Subaru’s schedule, replace on fault or performance concerns. Many workshops reassess sensors around 150,000–180,000 km, especially if fuel economy drops, the Check Engine light appears, or emissions tests flag issues.

Is it safe to drive with a bad oxygen sensor?
Short term, the car may run, but fuel use can spike and the catalytic converter can be put at risk if the mixture goes rich. It’s best to diagnose and sort it promptly to protect the cat and keep the Legacy running sweet.